Chapter 18 Lists

Like a string, a list is a sequence of values. In a string, the values are characters; in a list, they can be any type. The values in a list are called elements or sometimes items.

There are several ways to create a new list; the simplest is to enclose the elements in square brackets ([ and ]):

The first example is a list of four integers. The second is a list of three strings. The elements of a list don’t have to be the same type. The following list contains a string, a float, an integer, and (lo!) another list:

A list within another list is nested.

A list that contains no elements is called an empty list; you can create one with empty brackets, [].

As you might expect, you can assign list values to variables:

The syntax for accessing the elements of a list is the same as for accessing the characters of a string—the bracket operator. The expression inside the brackets specifies the index. Remember that the indices start at 0:

Unlike strings, lists are mutable. This means you can change an existing list. When the bracket operator appears on the left side of an assignment, it identifies the element of the list that will be assigned.

The one-eth element of numbers, which used to be 123, is now 5.

Figure 18.1 shows the state diagram for cheeses, numbers and empty.

Lists are represented by boxes with the word “list” outside and the elements of the list inside. cheeses refers to a list with three elements indexed 0, 1 and 2. numbers contains two elements; the diagram shows that the value of the second element has been reassigned from 123 to 5. empty refers to a list with no elements.

List indices work the same way as string indices:

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  Any integer expression can be used as an index.

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  If you try to read or write an element that does not exist, you get an IndexError.

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  If an index has a negative value, it counts backward from the end of the list.